doc/cephfs: fix prompts in fs-volumes.rst

Modernize prompts in example commands in file
fs-volume.rst and use a root prompt correctly instead
of using a non-privileged prompt when privileges
are needed.

Indent all JSON in example command outputs with
four spaces consistently instead of a mix of four and
two spaces.

Signed-off-by: Ville Ojamo <14869000+bluikko@users.noreply.github.com>